Hot runner manifold channel designis essential for the operation of an injection mold, impacting change-out time, part quality and even the number of possible drop locations in the mold. The key to advancing manifold design is having the ability to manufacture and implement curved channels in blocks of steel.
Cross-drilling and plugging a block of steel produces the simplest manifolds. Advanced manifolds may use brazed plug inserts to round internal corners or two-piece brazed manifolds to achieve curved internal channels without plugs. However, plugs are a complicated solution that can still leave drag spots. Brazed plates can be successful, but they have limited size and potential quality issues because of the weakness of braze joints (which are basically a metal “glue”). Manifolds are typically subject to the hottest temperatures and the highest pressure of either the mold or the hot half, making steel strength critical for maintaining the mechanical integrity of the hot half.
Curved manifold channel designproduced by metal additive manufacturing (AM) is a new method capable of achieving a better balance of fill across drop locations (or equalized residence time of the polymer), according to a recent research project by Oak Ridge National Laboratory (ORNL) and HTS International Corp. with the simulation effort headed by Dr. Prashant K. Jain and Dr. John A. Turner of ORNL. The findings in the research mean faster color changes, improved hot runner system stability and reduced pressure requirements. Hybrid metal additive manufacturing equipment has made it possible to produce smooth internal surface finish for H11, H13 and 420 SS variants of components. Hybrid AM involves a machining step during layer buildup, which offers superior surface finish to the standard printed finish from power-bed processes.
To date, metal additive manufacturing has produced advanced, curved manifolds H11 (1.2343) at a footprint of 600 millimeters by 400 millimeters. This is three to five times larger than the size of previous generations of curved manifolds produced using brazing technologies.
Curved manifold channel design produced by metal additive manufacturing is a new method capable of achieving a better balance of fill across drop locations.
The R&D began in 2017 in an effort to understand and publish guidelines for proper hot runner manifold channel design. These guidelines are relevant for hot runner manifold construction, independent of the method of manufacturing. Some of the basic findings are summarized here to showcase the considerable difference between a drilled, straight manifold design and a curved channel design.
Assumptions. This research was performed under the assumptions that polypropylene melts at uniform temperature, which is equal to 250°C (plus or minus 30°C) and that the polypropylene had a Newtonian, laminar flow. The pressure boundary was at an inlet of 10 kilopounds per square inch, and the research was performed using an isothermal steel manifold.
The curved manifold was designed for equal flow channel length. Other parameters could be optimized, including pressure drop and residence time. The results of the simulations were significant: residence time (or the time that the polymer spends in the manifold channels) was nearly equalized along the different flow paths. The residence time was cut from 400 milliseconds to 200 milliseconds in the outer channels , while the residence time was increased from 100 milliseconds to 200 milliseconds in the interior channels .
A look at the normalized, surface-concentration profile further validates the equal filling profiles in the curved manifold compared with the conventional manifold. While the cases shown represent two extreme ends of the manifold design spectrum, the benefits of removing constraints on the shape of the manifold channels are clear. For multi-cavity molders, new production capabilities with metal AM can enable the design of higher-density injection gates and can simplify the manifold stack. For single-cavity molders (as in many automotive molds), these new capabilities enable the strategic placement of drops in complex arrangements.
These benefits occur because patterns are developed without regard for intersecting cross-drill patterns, and curved channels can add path length into a reduced footprint for drops closer to the manifold inlet. Curved channels also reduce the path length for drops located further from the manifold inlet. Lastly, curving channels in a single manifold body reduce stack height instead of having to stack symmetrically drilled manifolds to achieve uniform channel length in high-volume tools.
In the recent global environment, mold makers and molders must look for ways to minimize cost and reduce production time. This is because they are interested in remaining competitive in the market. Suppliers of hot runners offer a wide variety of solutions to help address this phenomenon. This is done to overcome competitive pressures and meet the need of the customers at the same time. However, in this space, the system ranges from pre-manufacture, to configurable, and to completely customized systems. As a result, the difficulty arises from which hot runner will meet the need and be the most cost-effective. With a wide range of choices, how do we know the right system for a mold?
One of the major advantages of a hot runner is design flexibility. The ability to locate a wide variety of points on the part constitutes an interesting and desirable thing about a hot runner. The hot runner allows the gate to be placed at the most favorable location for part aesthetics and optimum filling. This is possible as a result of the usage of hot tip gating, edge gating, or valve gating. This flexibility offered by the hot runner also extends to the mold. It is evident where the flexibility in gate location helps to cool, optimize cavity orientation, and mold simplification.
The differences between hot runner systems and cold runners are quite heat stable and are used successfully in hot runner molds today.
Selecting a particular type of hot runner system is influenced by the product design and production requirements. There are many hot runner component and tool manufacturers available.
Manifold Design
Externally heated systems are best.
Internally heated manifolds are not suitable for TPEs. These systems typically have hot spots and stagnation zones that cause partially solidified material to cling to the cooler manifold walls.
Valve Gates
Valve gates offer the best solution for high production parts where surface quality is critical, such as medical and cosmetic products. Since valve gates leave only a slight ring on the part, gate vestige is minimized.
Further improvement can be obtained by positioning the valve recessed below the part surface or concealing the gate in the part detail for aesthetic products.
The gate diameter of a valve gate should be approximately 0.030″ to 0.125″, depending on the size and thickness of the part.
Valve gates do not require the material in the part to freeze before the valve is closed and hold pressure is released. Therefore, the screw recovery for the next cycle can start earlier and the total cycle time may be decreased. Valve gate elements need to be insulated from the mold plates to maintain proper temperature control.
Hot Tip Gates
The elements of the hot tip should be properly insulated from the mold plates and cavity. In order to achieve this, the land length of the gate may need to be lengthened and a portion of the land should be part of the cavity.
All passages within the tip should be highly polished and streamlined to minimize stagnation and degradation zones. The efficiency of the design may be verified by recording the time it takes to make a complete color change while producing parts. This demonstrates whether there is any residual dead zone material that continues to enter the melt stream.
For hot tip gate systems, there should be a delay long enough for the part to set up completely before mastication is initiated for the next cycle. Without a delay, the parts may become overpacked. This is particularly important for low hardness, high flow materials. To reduce overpacking for thick-walled parts with large gates, use minimal back pressure during mastication.
The hot runner offers a flow advantage in the sense that the melt channels are in separate externally heated manifolds These channels are well insulated from the surrounding mold plates. In comparison with the cold runner plate, the hot manifold can be designed to have flow channels on multiple levels. This helps to ensure that the resin flows at the same length from the nozzle to each cavity. This flow ensures the same channel profiles and diameter, the same number of turns, heat soak, and pressure loss. In essence, the plastic reaches each cavity at the same thermal history and pressure irrespective of their numbers. This in-built pressure and temperature control are beneficial for resins that exhibit narrow processing windows.
The hot runner mold is a mold that uses a heating device to keep the melt in the runner from solidifying. The plastic raw materials will enter the mold through the material hall, and the product will be formed in the mold. When the cross-sectional area of the gate is larger, the space for the plastic to flow is larger, and the flow resistance will also be generated. The flow channel can be divided into circular, semi-circular, trapezoidal, and so on.
Advantages of Hot Runner:
Shorten the cycle –
The molding cycle of the part is shortened. Because there is no limitation on the cooling time of the runner system, the part can be ejected in time after it is formed and solidified. Many thin-walled parts produced with hot runner molds can have a molding cycle of fewer than 5 seconds.
Save plastic –
There is no cold runner in the pure hot runner mold, so there is no production cost. Hot runner technology is an effective way to reduce material costs.
Reduce waste –
Reduce waste and improve product quality. During the hot runner mold forming process, the plastic melt temperature is accurately controlled in the runner system. Plastic flows more uniformly into the cavities, resulting in consistent quality parts. The gate quality of the parts formed by the hot runner is good, the residual stress after demolding is low, and the deformation of the parts is small.
Production automation –
Elimination of subsequent processes is conducive to production automation. The finished product is the finished product after being formed by the hot runner mold, and there is no need to trim the gate and recycle and process the cold runner. Conducive to production automation. Many foreign product manufacturers have combined hot runner and automation to greatly improve production efficiency.
Disadvantages of Hot Runner:
Rising costs –
Hot runner components are more expensive, and the cost of hot runner molds may increase significantly. If the output of parts is small, the proportion of mold tool cost is high, and the expensive hot runner system is the main key to the widespread use of hot runner molds.
High equipment requirements –
The production process equipment requirements are high, and the hot runner mold needs precision machining machinery to ensure. The integration and cooperation requirements of the hot runner system and the mold are extremely strict, otherwise, there will be many serious problems in the production process of the mold. For example, poor plastic sealing leads to plastic overflow and damage to hot runner components and interrupts production, and the relative position of nozzle inserts and gates is not good, resulting in a serious decline in product quality.
Complex operation and maintenance –
Compared with cold runner molds, hot runner molds are more complicated to operate and maintain. Improper use and operation can easily damage hot runner parts, making production impossible and causing huge economic losses. For new users of hot runner molds, it takes a long time to accumulate experience.
For a balance hot runner deisgn, Design runner system with same pressure drop at each injection point. Different flow path lengths are compensated for cross section of flow path, balance calculated by principle of rheology is provided, which is called rheological balance, flow distance from main nozzle to gate of each nozzle is different, different flow length ratios will lead to differences in filling pressure of melt of each injection gate. However, after rheological calculations and adjusting radius of each runner, balanced filling of each gate can also be achieved.
Some suggestions on rheological balance design of a multi-cavity unbalanced runner system:
(1) Unbalanced design of subtle parts of runner system with one mold and multiple cavities can achieve rheological balance through simple calculations, shorten length of runner, reduce stratification of runner, and compress thickness of runner plate. As a result, the overall thickness of hot runner part of mold is improved.
(2) For completely unbalanced design of runner system with multiple cavities, flow balance equation can be used for preliminary calculation and design, then rheological balance method can be used for correction, so that pressure of each gate is basically equal.
(3) Balance design of a runner system with multiple cavities also needs to design runner size according to principle of rheological balance, so that shear rate and shear force of plastic melt in manifold pipes at all levels are basically unchanged, so as to achieve uniform temperature distribution of entire runner plate, and there is no local high temperature that affects flow of plastic melt.
(4) Non-equilibrium design of multi-cavity runner system reaches rheological equilibrium. After rheological equilibrium is reached, melt shear rate and shear force in various runners will change, and transmission of plastic melt will cause instability. In order to prevent such a situation, it is necessary to calculate in advance fluid shear rate limiting flow passages of each section.
(5) Flow balance design at all levels, calculate the best shear rate and allowable pressure loss, you can use DIMH hot runner system design program to optimize design of runner system (Design of Injection Mold for Hot Runner System).
Hot runners offer many advantages compared to coldrunners, especially for multicavity molds. Hot runnersimprove productivity by reducing cycle time, materialscrap, clamping force and secondary operations likeremoving the gate and regrinding the runners. Some hotrunner applications may present some drawbacks such asthe premium cost and lengthy color change times. If thehigher cost can be offset by the increase in productivity,color change time remains a concern for multicavity hotrunner molds that perform frequent color changes. Oneway to encompass the benefits of both hot runners and coldrunners in one runner system is through an insulatedrunner. Molds using insulated runners achieve a one-shotcolor change by ejecting the runner filled with the old colorbefore introducing the new color. However, insulatedrunners have limited application in terms of resin, runnerlayout and cavitation
Prior to hot runner technology, cold runners were widely used on injection molds. Cold runner molds faced many challenges in conveying the resin from machine barrel to cavities without affecting the flow and thermal characteristics of the resin. With the advancement of resin types and the complexity in mold and part designs, it became more and more difficult to control the molding process via cold runner molds to produce molded parts of acceptable quality.
However, with the introduction of hot runner technology with advanced thermal controls, processing of wider ranges of resin became more practical and convenient to injection molders. Unlike a cold runner mold, the hot runner components are individually heated to ensure the resin maintains the temperature continuously through the mold. The temperature of each hot runner heated component can also be precisely controlled to ensure the process is optimized to the requirements of each type of resin delivering the highest possible part quality. Today, hot runners are capable of producing highly complex parts in a wide range of sizes which are utilized in every industry.
Hot runner technology, introduced to the plastics industry over 50 years ago, revolutionized injection molding processing capabilities by improving molded part quality, enhancing operational efficiencies, reducing scrap and saving money.
what is a hot runnerA hot runner system is a molten plastic conveying unit used within an injection mold. In other words, a hot runner system consists of heated components (generally via electricity) used inside the plastic injection molds, which brings the molten plastic from the barrel of an injection molding machine into the cavities of the mold. The sizing of hot runner melt channels depends on many factors such as the type of resin, the injection speed, fill rate, and the molded part. A temperature controller (standalone controller or controls from the injection molding machine) heats the hot runner system within the injection mold and the resin inside the machine barrel to processing temperature and injects the resin into the mold. The resin travels through the inlet, down into the manifold which then splits to the various nozzles and through injection points (or gates) into the final mold cavity where the final part is formed. Today’s molds can have anywhere from 1 to over 192 nozzles depending on the plastic parts being manufactured.